The Ireland captain guided his side to victory in their opening game in Group C but they were less than convincing at rain-swept Stadium Taranaki.
Wing Tommy Bowe overcame an error-strewn start amid poor conditions to score a try in each half, while hooker Rory Best also crossed.
The United States had the final say when centre Paul Emerick ran in a late intercept try.
O'Driscoll told ITV1: "We didn't play well, we had to fight for our win.
"I suppose we got the win but we have a hell of a lot to improve on.
"We couldn't get any continuity. We were a little bit loose."
He added: "It's good to get 80 minutes under your belt now we've got a big challenge at the weekend against Australia.
"We can work on bad aspects of our game during the week."
Coach Declan Kidney admitted Ireland had not been particularly impressive but insisted he was "delighted with the win".
Kidney also looked forward to the next game against Australia, saying: "Another big one - that is why you get into it.
"It's a six-day turnaround to Eden Park."
As for areas he can identify for attention, Kidney said: "Maybe the man above makes it a bit dryer and we can play a little more ball and we can bring more variation into our play."
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